In recent years, cognitive radio systems that detect and use an unused frequency band have been studied and developed. Usable frequency bands change with time and location due to communications by primary users, the mobility of terminals, or other reasons. Therefore, spectrum sensing for detecting unused frequency bands is one of the key techniques in cognitive radio systems.
As unused frequency band detection methods, there have been known methods disclosed in patent literatures 1 to 5. Patent literature 1 describes a technique in which base stations using TV frequency bands according to e.g. IEEE 802.22 sense unused frequency bands in cooperation. Patent Literature 2 describes a plurality of nodes that scan respective predetermined portions of spectrum and share the scan results among them. Patent Literatures 3 to 5 describe performing a first coarse scanning over all the frequency bands and performing a second scanning with high accuracy based on the result.